The induction of low-dose paralysis in non-responding mice by prior treatment with a sub-immunogenic dose of SSS-III was found to produce significant suppression of the antibody response of transferred spleen cells from histocompatible responding mice; since this form of unresponsiveness has been shown to be antigen-specific and T cell dependent, it is most likely due to the activity of suppressor T cells, rather than the inhibitory effects of antigen-antibody complexes or antibody-mediated feedback inhibition. Treatment with antilymphocyte serum, at the time of immunization with ALS, appeared to inactivate suppressor - but not amplifier - T cells. The basis for this "apparent" differential sensitivity was found to be due mainly to the times after immunization at which these regulatory T cells are activated; precursors of both suppressor and amplifier T cells were not inactivated by treatment with this T cell depleting agent.